11,000 rpm angle grinders are usually small compact angle grinders, with a max wheel size of 4.5”(115mm), although 4”(100mm) angle grinders also may run at 11,000 rpm.
4” angle grinders are pretty obscure in the USA. Makita sells at least one model, but Makita is a Japanese manufacturer and 4” angle grinders are a standard and still common size in Japan. Harbor Freight also sells a 4” model. I presume in the past other manufacturers like Hitachi might have as well.
For 4.5” angle grinders, the common US speed was 10,000 rpm, and that speed for 4.5” angle grinders was pretty standard.
Bosch manufactured some of their 4.5” compact angle grinders in 11,000rpm, as did Fein, and maybe it’s become somewhat more common since I started buying tools 2+ decades ago, but it was usually the exception as far as speed for a 4.5” rather than the norm.
Fein does seem to manufacture most of their current smaller (4.5” and 5”) angle grinders in 11,000 rpm nowadays do maybe it’s a better speed, but I’m not certain.
As far as 8,500-9,000rpm grinders, those are and were commonly made for running 6” wheels, and 6” angle grinders were always the weird size, at least for the last two decades, within the US. Fein and Flex and Metabo offered one or two 6” models, as did Porter Cable (whose grinders were made by Flex) But 6” angle grinders were sort of a weird soecialty size.
The other reason you find 8,500rpm grinders is Inox( ie. stainless) models, meant for people who process stainless steel.
Speeds typically vary, but grinders meant for stainless steel processing usually come in 5” size, and run somewhere around 2,000rpm-7,600rpm, and almost always have variable speed.
Maybe I’m a bit behind, since angle grinder speeds seem to have increased over the past 5 years or so with newer models,
Or maybe cutting and grinding wheels have gotten safer and stronger, since since specified max rpm of the grinding wheel usually specifies the max speed of the grinder you should use.
If you want higher torque, just check the angle grinder wattages of the grinders you’re considering.
Efficiency of motors can vary by manufacturer, but from the same manufacturer, a 5” 1200 watt grinder is going to have higher torque than a 5” 700watt angle grinder.
The only exception to this is possible differences in motor design, such as the newer Hitachi/MetaboHPT brushless grinders.
Two other things to consider, are the guard size, and possible safe use of grinder wheels.
A larger grinder guard can let you use larger grinding wheels, as well as still letting you use smaller sizes, but the larger grinder guard may get in the way during cuts in confined places, as well as if you need to grind close to or next to an obstruction.
The other major issue with rpm is safety.
Even with “variable speed” angle grinders, you’re really not supposed to use wheels rated above the max rpm on the grinding disc/wheel. Thr reason being that if the electronic speed control fails, the grinder can ramp up to full speed and possibly cause a lower rated disc to explode.
A lower 8,500 rpm would prevent this.
You might also check into whether it might be possible to use a 6” guard on the grinder if you’re considering buying a 8,500rpm grinder.
Manufacturers won’t tell you if it’s possible, but if you go to ereplacement parts, you can use their database to check whether the angle grinder lower section were the guard attaches is used on more than one model, and then check whether those models include both 5” and 6” grinders. If so, than you can probable purchase an extra larger guard and use it with the larger wheels if the max speed of the wheels is within spec.
